Adsorption of contaminants from gaseous stream and in situ regeneration of sorbent

ABSTRACT

Contaminant is removed from a gaseous stream, especially an air stream bearing the contaminant, by adsorption on a sorbent which is a resiliently compressible, electrically conductive, activated carbon cloth material, leaving a gaseous stream liberated of the contaminant; the carbon cloth material loaded with the contaminant may be regenerated by desorption of the contaminant; the carbon cloth material loaded with contaminant is housed in a vacuum and electric current is passed through the carbon cloth material generating heat in the cloth material which is effective to desorb the contaminant which is exhausted under vacuum; the level of heat generated is varied as required, typically to develop a temperature of 250 to 500° C. in the cloth material, by varying the compression of the cloth material; desorption is typically achieved in about 30 minutes at a vacuum of 10 Torr or less.

This application claims the priority benefit of PCT/CA02/01915, filedDec. 11, 2002, which claims the priority benefit of U.S. provisionalapplication Ser. No. 60/340,874, filed Dec. 19, 2001, which isincorporated herein by reference.

TECHNICAL FIELD

This invention relates to a method and apparatus or device for removinga contaminant from a gaseous stream bearing the contaminant; theinvention also relates to a method and apparatus or device for desorbinga contaminant from a carbon cloth material loaded with adsorbedcontaminant.

More especially, the invention relates to the removal of gaseouscontaminants from an air stream by an activated carbon cloth sorbent andthe subsequent “in-situ” regeneration of the carbon cloth by the directapplication of an electric current while the electro thermally heatedcarbon cloth is maintained under vacuum and the concomitant collectionof the contaminants or their thermal degradation products from theevacuated air for recovery or disposal. This invention is well suitedfor the purification of air in enclosed spaces such as aircraft cabins,submarines, vehicles, buildings, private residences and personal airrespirators.

BACKGROUND ART

Activated carbon is widely used today as a filtration medium in industryand elsewhere for the removal of gaseous contaminants from gaseous andliquid streams where they constitute less than 1% of the fluid stream.The demand for this material is estimated at 220,000 metric tonnes peryear and increasing at the rate of 5.4% per annum through 2002. This isdue, in part, to increases in the output of chemical processes and morestringent environmental regulations worldwide. For instance, theenvironmental Protection Agency (EPA) has lowered emissions standardsfor several environmental pollutants. It should be noted that theemissions of volatile organic contaminants in the U.S.A. in 1998 totaled1.62×10⁷ Kg and solvent utilization accounted for 30% of theseemissions.

In addition, the occupants of office buildings, the residents of privatehomes and institutions, the passengers in commercial aircrafts, trainsand vehicles are increasingly concerned about the quality of the airthey breathe. These concerns have become more acute with theimplementation of energy conservation measures in these microenvironments, and the increased usage of outgasing synthetic materials.This has led to an increased interest in ventilation systems capable ofcontrolling the presence of gaseous contaminants in breathing air. Suchsystems invariably make use of activated carbon to control thesepollutants.

Most of the activated carbon used today for the removal of gaseouscontaminants from air streams at concentrations <1% is either granulatedor pelletized activated carbon or powdered activated carbon (PAC)usually placed in trays. The contaminated air stream is routed through abed of activated carbon which adsorbs the gaseous contaminants. Thepurified air stream is either recycled or discharged to the atmosphere.Inherent problems associated with such systems include high pressuredrops and the periodic replacement of the spent carbon, a laborintensive, potentially hazardous and costly procedure. Alternatively,carbon can be incorporated in a matrix or bonded to fiber and shaped aspanels, blocks or slabs as described in (WO 94/03270). Although thisaddresses the problem of high pressure drop across the filtrationmedium, it leads to a decrease in adsorptive capacity of the bondedcarbon and the need for periodic replacement remains.

Activated carbon is also available as activated carbon cloth both wovenor knitted, and as a felt. It can be used to make very thin carbon bedshaving very low pressure drops and with an adsorptive capacityequivalent to deeper granular carbon beds. It is ideally suited to airpurification. However, as for the other forms, it must periodically bereplaced, and the time between replacements can be comparatively short.

The spent carbon, in all its forms, is either regenerated or replacedwhen its effectiveness falls below an acceptable value. Replacement withvirgin activated carbon is costly and laborious. Movement of the spentcarbon off site for regeneration incurs transportation and labor costsand degradation of the medium. The process of regeneration of theadsorptive capacity of the spent carbon invariably involves the heatingof the carbon beds. This heat is usually supplied externally by the useof hot air or steam, or by placing hot elements in the carbon medium(U.S. Pat. No. 5,187,131). The regeneration gases burn the carbon mediumwith a concomitant loss in both the amount of active carbon and theadsorptive capacity. Similar procedures are available for on-siteregeneration of spent carbon.

Regeneration of spent carbon by means of vacuum procedures have beenlargely ineffective to date, because adsorbate volatilization requiresthermal energy and vacuum desorption has a chilling effect on the carbonwhich cannot be offset because the vacuum environment is known to be avery efficient thermal isolator. Hence, for all available processes todate, heat energy cannot be added to the chilled carbon during vacuumadsorption. At best, the process requires an inordinate amount of timeto achieve acceptable regeneration.

In general, for all the aforementioned regeneration procedures, theregenerated activated carbon never attains its original adsorptivecapacity because there is a residual adsorbate which resists removal.This is especially common with “in-situ” steam regenerated activatedcarbon. The result is that eventually the regenerated carbon does notsatisfy contaminant removal requirements, and must be replaced.

It is known in the art that activated carbon is capable of conductingelectricity. The resistance properties of this material are such thatuseful heat can be generated in this manner (U.S. Pat. No. 6,107,612).Attempts have been made to generate the heat required for regenerationof spent carbon by making use of this property of carbon, (DE 4104513).However, only limited successes have been achieved with this procedurewhen applied to granulated/pelletized/powdered carbon, because ofnon-uniform heating patterns, hot spots and short circuits.

Better results have been obtained by passing an electric current throughthe carbon cloth thus generating the heat required for desorption withinthe sorbent medium itself where the thermodynamics of the adsorption anddesorption processes apply (U.S. Pat. No. 5,912,423) This method isinherently more thermally efficient than prior art methods. However, themethod requires that a purge stream of air or inert gaseous be used toconvey the desorbed contaminants away from the cloth. The use of air isnot recommended since it invariably leads to significant loss of carbonover time by oxidation; and the method requires the use of large volumesof inert gaseous during the regeneration phase which may take threehours or more depending on loading and adsorbate characteristics. Thismethod also requires that technically sophisticated means, usuallycryogenic, be used to remove the contaminants from the large volume ofinert gases used.

DISCLOSURE OF THE INVENTION

It is the object of the present invention to provide means and a methodfor removing gaseous contaminants from a gaseous stream, especially anair stream, without the disadvantages associated with the prior artmethods.

It is a further object of the invention to provide a method to removegaseous contaminants present in gaseous or air streams using activatedcarbon cloth material as sorbent that can be regenerated “in-situ”effectively, uniformly and very rapidly by passing an electric currentthrough the carbon cloth material under vacuum.

A further object of the invention is to provide a method and apparatusfor the effective recovery of evacuated contaminants for disposal orprocessing.

This invention also provides for the continuous removal of contaminantsfrom a gaseous stream, especially an air stream, on a sorbent, andsubsequent rapid non-destructive regeneration of the sorbent. Theinvention provides for a sorbent bed having minimal pressure drop and alengthy life expectancy, the original adsorptive capacity of which isessentially restored by regeneration, and where bed depth is limitedonly by the capacity of the host system.

In accordance with one aspect of the invention, there is provided amethod for removing a contaminant from a gaseous stream bearing thecontaminant comprising:

-   -   i) flowing the gaseous stream bearing the contaminant into a        chamber housing a resilient compressible electrically        conductive, activated carbon cloth material,    -   ii) adsorbing said contaminant on said cloth material with        formation of a carbon cloth material loaded with said        contaminant, and a gaseous stream liberated of said contaminant,        and removing said gaseous stream from said chamber; and

discontinuing the flow of gaseous stream bearing the contaminant intothe chamber, placing said chamber under vacuum and passing an electriccurrent through said carbon cloth material loaded with said contaminantto generate heat in said carbon cloth material effective to desorb saidcontaminant from said carbon cloth material, while maintaining saidvacuum.

In accordance with another aspect of the invention, there is provided amethod of desorbing a contaminant from a resiliently compressible,electrically conductive, activated carbon cloth material loaded withadsorbed contaminant comprising:

-   -   a) housing said carbon cloth material loaded with adsorbed        contaminant in a chamber,    -   b) placing said chamber under vacuum, and passing an electric        current through said carbon cloth material and generating heat        in said carbon cloth material effective to desorb said        contaminant from said carbon cloth material.

In accordance with still another aspect of the invention, there isprovided a device for removing a contaminant from a gaseous streambearing the contaminant comprising:

-   -   a) a chamber housing a resiliently compressible, electrically        conductive, activated carbon cloth material;

b) said chamber housing a first port for introduction into said chamberof a gaseous stream bearing contaminant and a second port for removalfrom said chamber of the gaseous stream liberated of the contaminant;

-   -   c) a pair of spaced apart electrodes in said housing, said        electrodes being electrically in contact with said carbon cloth        material for flow of electric current between said electrodes        and through said carbon cloth material, said carbon cloth        material generating heat on passage of said electric current        therethrough, and

said chamber being gas-tight to support a vacuum therein during thegeneration of heat by the carbon cloth material.

In accordance with yet another aspect of the invention, there isprovided a device for desorbing a contaminant from a resilientlycompressible, electrically conductive, activated carbon cloth materialloaded with adsorbed contaminant comprising:

-   -   I) a chamber housing said carbon cloth material,    -   II) a pair of spaced apart electrodes in said housing, said        electrodes being electrically in contact with said carbon cloth        material for flow of electric current between said electrodes        and through said carbon cloth material, said carbon cloth        material generating heat on passage of said electric current        therethrough,    -   III) said chamber being gas-tight to support a vacuum therein        during the generation of heat by the carbon cloth material, and    -   IV) an exhaust port for exhausting under vacuum a contaminant        desorbed from the carbon cloth material by the heat generated by        the carbon cloth material.

DESCRIPTION OF PREFERRED EMBODIMENTS

a) Adsorption and Desorption

The invention contemplates adsorption of contaminant from a gaseousstream bearing the contaminant on an activated carbon cloth materialwhich is electrically conductive and resiliently compressible.

The invention also contemplates desorption of the contaminant from thecloth material loaded with adsorbed contaminant.

The adsorption is typically carried out at ambient temperatures of 10 to30° C., more especially about 20° C. by flowing a gaseous stream bearingthe contaminant through the activated cloth material; the contact timefor adsorption is typically less than 1 second and more especially lessthan 0.1 second.

The contaminant may be a useful material which may be recovered forre-use in a subsequent step, for example, solvent which accumulates inthe atmosphere in industrial premises; or it may be a noxious ornon-useful material which is to be collected for disposal. In general,the contaminants will be organic substances which vaporize at moderatetemperatures, for example industrial solvents, or organic or inorganicgases or fine particulate organic or organometallic substances whichvaporize at elevated temperatures or thermally degrade at elevatedtemperatures to discharbeables gases.

The desorption is carried out by heat generated in the carbon clothunder vacuum, which heat vaporizes the contaminants. The heat isgenerated by passing electric current through the carbon cloth and thelevel of heat generated can be varied by compression or decompression ofthe carbon cloth or by passing a larger or smaller electric currentthrough the carbon cloth; or both. Thus, the heat or temperature can beselected depending on the contaminant or contaminants to be desorbed andthe desired rate of desorption. Higher temperatures result in more rapiddesorption, however, contaminant thermal decomposition or degradation ismore likely at higher temperatures.

It is within the scope of the invention to generate the heat at levelswhich are effective to thermally degrade organic contaminants to simplemolecules such as carbon dioxide, nitrogen and water which can bedischarged to the atmosphere, or simple molecules such as ammonia andsulphur dioxide which can be discharged to atmosphere or recovered.

In general desorption is readily achieved at a temperature, developed bythe carbon cloth under a vacuum of, not more than 10 torr and morepreferably less than 5.0 torr, more preferably less than 1 torr and mostpreferably less than 0.5 torr, of 250° C. to 500° C., more especially300 to 350° C., and at such temperatures desorption is completed in 20to 45 minutes and typically less than 30 minutes.

In the case where the contaminant is to be thermally degraded,temperatures of the order of 1000° C. or more are readily developed inthe carbon cloth by adjustment of the level of compression, or varyingthe current flowing through the carbon cloth.

The desorption of contaminant under vacuum from the carbon clothmaterial regenerates the activated carbon cloth, essentially to itsoriginal adsorptive capacity. The operation in a vacuum avoidsdegradation of carbon cloth by air and permits use of lower heatingtemperatures for desorption which similarly avoids degradation of thecarbon cloth exhibited at high temperatures.

b) Carbon Cloth Material

The adsorbent or sorbent employed in the invention is a carbon clothmaterial. The carbon cloth material is electrically conductive and notonly achieves a wide range of heat generation on compression, but largeheat increase is achieved rapidly on application of relatively minorchange of compressive force.

In particular, the cloth material which may, for example, be woven,non-woven, knitted or felted may be carbon cloth, carbon felt, carbonimpregnated cloth or graphitized carbon cloth.

The carbon cloths and felts are formed by carbonizing cloths and feltsof organic fibers, filaments, monofilament yarns and multi-filament yamswhich may be synthetic, for example, polyacrylonitrile fibers, filamentsor yarns, or natural, for example, cotton or carbon pitch.

Carbon cloths deteriorate in the presence of oxygen at temperaturesabove 400° C. In applications where these cloths are employed fordevelopment of temperatures above these levels, chemical treatment ofthe cloths to inhibit oxidation, may be necessary, inert (non-oxygen)gas bearing contaminants do not face this shortcoming.

Heat is generated in the carbon cloth material by the passage ofelectric current between the electrodes and through the carbon clothmaterial, and this heat serves to desorb contaminant from carbon clothwhich has been previously loaded with the contaminant by adsorption froma gaseous stream bearing the contaminant.

The expression “resiliently compressible” is to be understood asindicating that the carbon cloth material can be compressed byapplication of a compressing force thereto and that the material expandsor is restored to substantially its pre-compression state on removal ofthe compressing force. In part the cloth material may be considered tohave an elastic memory of the pre-compression state so that it can becompressed repeatedly to different levels of compression but restored tothe initial state or to a less compressed state on release or partialrelease of the compressive force.

It will be understood that the resilience or ability of the compressedheating element to relax to its pre-compression state may be altered byage.

References to varying the compression of the heating element contemplatedecreasing the compression, i.e., decompressing so as to increase theheat generated and increasing the compression, i.e., compressing todecrease the heat generated.

The carbon cloth material may be in a partially compressed stateinitially to effect a desired initial level of heating for a particularelectric current. The carbon cloth material may then be subjected todecreasing levels of compression to increase the level of heatgenerated.

The cloth material should suitably withstand the heat which it generatesat temperatures to be developed, and not degenerate on continuous,continual or repeated exposure to such heat. The carbon cloth materialmay, in particular, comprise a plurality of layers of electricallyconductive carbon cloth in adjacent side-by-side relationship, eachlayer of the plurality is in electrical contact at least with anadjacent layer and in particular a major face of each cloth layer is inelectrical contact with a major face of an adjacent cloth layer, thecloth layers forming a stack bed or pile in which the layers are inopposed facing relation. The stack or pile need not, however, bedisposed such that the layers are horizontal and any disposition of thelayers from horizontally oriented to vertically oriented is possible.

The plurality of layers may be formed from discrete separate layerswhich may be the same or different, or may be formed from a continuouslength of cloth folded repeatedly in concertina fashion to produce theplurality of layers or may be formed from combinations of separatelengths folded in concertina fashion and stacked together orcombinations of separate lengths folded in concertina fashion anddiscrete separate layers, stacked together.

c) Compressive Force

In especially preferred embodiments the resiliently compressible,electrically conductive cloth material comprises a plurality of layersof carbon cloth disposed in a separating space between a pair of spacedapart electrodes such that the outermost layers of the plurality oflayers are in electrical contact with the electrodes, thereby providinga path for flow of electric current between the electrodes.

In this case, one or both of the electrodes is adjustably positionableto alter the distance separating the electrodes, i.e., the length of theseparating space. Adjusting the position of one or both of theelectrodes to vary the distance separating them provides a compressiveor decompressive force on the plurality of cloth layers.

It is also possible, however, to dispose the layers between a pair ofspaced apart insulated members, for example, ceramic members, withelectrical connectors extending from the electrodes through theinsulated members to make electrical contact with the cloth layers ordirectly to the outer cloth layers. In this case adjusting the positionof one or both of the insulated members to vary the separating spacebetween the insulated members provides the required compressive ordecompressive force.

It will be understood that the electrodes or insulated members must havea structural integrity capable of applying the compressive force to thecloth layers.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of a device of the invention for use incarrying out the methods of the invention;

FIG. 2 illustrates schematically a detection system for use in themethod of adsorbing contaminants of the invention;

FIG. 3 illustrates schematically apparatus for collection of desorbedcontaminant in accordance with the invention;

FIG. 4 illustrates graphically the relationship between powerconsumption and vacuum in the chamber of the device of the invention;

FIG. 5 illustrates graphically the relationship between the equilibriumtemperature of carbon cloth material in a device of the invention andthe amount of adsorbate (toluene) removed in a 30 minute period; and

FIG. 6 illustrates graphically the relationship between the amount ofdesorption (toluene) and the desorption time in a device of theinvention.

DESCRIPTION OF PREFERRED EMBODIMENT WITH REFERENCE TO THE DRAWINGS

With further reference to FIG. 1, a device 10 of the invention has achamber 12 housing a sorbent bed 14. Opposed ends of the bed 14 are inelectrical contact with copper electrodes 16 and 18 respectively.

Chamber 12 has an inlet port 20, an outlet port 22 and vacuum valves 24and 26.

Sorbent bed 14 is composed of a plurality of activated carbon clothlayers 28 in side-by-side relationship, with adjacent layers 28 inelectrical contact, the resulting sorbent bed 14 being resilientlycompressible, and disposed between graphite cloth layers 40. Sorbent bed14 is housed within a ceramic sleeve 30.

A thermocouple 32 measures the temperature in the sorbent bed 14.

Chamber 12 further includes a pressure transducer 34 and a vernier 36 toadjust the spacing between copper electrodes 16 and 18, thereby varyingthe degree of compression of the carbon cloth layers 28 of sorbent bed14.

Outlet port 22 is connected to a source of vacuum 38 (not shown).

With further reference to FIG. 2, there is illustrated a system 50employing a pair of devices 56 and 58 of the invention in parallel.Device 56 being in use for adsorption of contaminants and device 58being under regeneration of the activated carbon cloth.

System 50 further includes a contaminated gaseous stream line 52 and aclean gaseous stream line 54. A sensor 60 is disposed in line 54.

The devices 56 and 58 are particularly of the same form as device 10illustrated in FIG. 1.

With further reference to FIG. 3, a contaminant recovery assembly 70 foruse in conjunction with the device 10 of FIG. 1 during the regenerationof the carbon cloth includes a recovery container 72 and a vacuum pump74.

Contaminant inlet line 76 in operation would communicate with outletport 22 of device 10 in FIG. 1.

Assembly 70 further includes vacuum valve 78 and container 72 furtherincludes a pneumatically activated piston 80 and a liquid contaminantoutlet 82.

In operation of the device 10 of FIG. 1, a gaseous stream bearingcontaminant flows through inlet port 20 into chamber 12 and passesthrough the sorbent bed 14, the contaminant being adsorbed from thegaseous by the activated carbon cloth layers 28; gas liberated ofcontaminant flows out of chamber 12 through outlet port 22. Thisadsorption process may be carried out at ambient temperature.

Thus, by reference to FIG. 2, the gas liberated of contaminant whichleaves chamber 12 through outlet port 22, flows into clean gaseousstream line 54, the contaminant being retained in sorbent bed 14 whicheffectively is in the device 56 of FIG. 2. By further reference to FIG.2, the contaminated gaseous stream thus flows through device 56 but notthrough device 58. The level of contaminant in the gaseous stream inclean gaseous stream line 54 is monitored by sensor 60.

When sensor 60 detects an unacceptable level of contaminant indicatingthat the effectiveness of the sorbent bed 14 in the device in use 56,has been reduced as a result of loading of the bed 14 with contaminant,the flow of contaminated gaseous stream into device 56 is discontinued.

During the period of use of device 56, device 58 has been underregeneration of the sorbent bed 14. Device 58 is now brought into useand the contaminated gaseous stream is fed through device 58 for removalof contaminant and the gase stream liberated of contaminant flows toclean gaseous stream line 54 where the level of contaminant is monitoredby sensor 60. During this operation, device 56 which is no longer inuse, is subjected to the regeneration of the sorbent bed.

The regeneration is further described with reference to FIG. 1.

In carrying out the regeneration, inlet port 20 is closed and outletport 22 is connected to a source of vacuum 38, and the chamber 12 isevacuated. Electric current is fed between copper electrodes 16 and 18and passes through the carbon cloth layers 28 of sorbent bed 14, whichlayers 28 are loaded with contaminant previously adsorbed from thegaseous stream. The passage of electric current generates heat in thecarbon cloth layers 28, and the level of heat can be varied by varyingthe compression on the carbon cloth layers 28. The compression is variedby varying the distance separating electrodes 16 and 18, this distancebeing altered by moving electrode 16 relative to electrode 18 by meansof the vernier 36.

This latter operation of varying the level of heat generated is fullydescribed in U.S. Pat. No. 6,107,612, the teachings of which areincorporated herein by reference.

The level of heat generation is adjusted as described to achieve a levelappropriate for desorption of the contaminant, under the vacuum inchamber 12 from the cloth layers 28. In general, a temperature in therange of 300° to 500° C. is sufficient for most contaminants, under avacuum of 10 Torr or less, and desorption of the carbon cloth layers 28is completed in 20 to 45 minutes.

The contaminants are desorbed in a gaseous or vapor state depending onthe temperature generated. If desired, heat generation can be effectedto a high temperature for thermal degradation of the contaminant tosimple gaseous molecules which can be discharged to atmosphere, in thosecases where it is not desired to collect the contaminant for reuse orfor disposal.

The desorbed contaminant or the thermal degradation product is exhaustedfrom chamber 12, under vacuum, through outlet port 22.

With further reference to FIG. 3, there is illustrated a contaminantrecovery assembly 70 for use in conjunction with device 10 of FIG. 1 forthe case in which the contaminant is being recovered rather thanthermally degraded, either for reuse or disposal.

The contaminant exiting chamber 12 through outlet port 22 of FIG. 1 isdrawn by vacuum pump 74 along contaminant inlet line 76 and intorecovery container 72. Container 72 can be sealed from inlet line 76when desired. Contaminant vapors collected in recovery container 72 canbe compressed to liquid form by pneumatically activated piston 80 andultimately the resulting contaminant liquid can be discharged fromoutlet 82 either for disposal or for reuse in industrial processes.

GENERAL TECHNICAL DESCRIPTION

As described above, FIG. 1 is a schematic cross-section of an apparatusfor the adsorption and desorption of gaseous contaminants in a gaseousstream of the invention.

It is constructed entirely with commercially available components usedfor low vacuum work, that is, with flanged piping having seals allowinglow pressures (<0.1 mmHg) to be reached without leakage. The sorbent bed14 is contained in ceramic sleeve 30 between electrodes 16 and 18 whichsuitably are copper ring electrodes, however, conductive metal used isdetermined by the characteristics of the contaminant gaseous stream. Forinstance, a corrosive gaseous stream would require a metal such asstainless steel.

The sorbent bed 14 is contained between non-adsorptive graphite clothlayers 40 that ensure even distribution of electrical current to the bed14. Thermocouple 32 in the sorbent bed 14 monitors the temperatureduring regeneration and pressure transducer 34 monitors the pressure inthe chamber 12 during this process. Vernier 36 or a similar lesssophisticated device such as a screw allows the operator to select thedistance between the electrodes 16 and 18 to attain the desiredregeneration temperature (U.S. Pat. No. 6,107,612, Farant). Thetemperature of the sorbent bed 14 can also be varied by increasing thecurrent applied to the bed 14 and keeping the electrodes 16 and 18fixed. It should be noted however, that the electrode distance willstill have to be reset periodically due to the reduction in the depth ofthe sorbent bed 14, as a result of shrinkage, following extended usage.

Adsorption process

The properties of the sorbent bed 14 used are dictated by the nature ofthe gaseous contaminants to be removed from the gaseous stream, that is,the pore size distribution of the sorbent bed 4 is pre-selected forparticular conditions. The adsorptive capacity of the sorbent bed 14 andits bed depth are also selected according to the anticipated range ofcontaminant concentrations and the gaseous flow rate and desired timebetween regeneration phases.

The contaminated gaseous stream, at room temperature, is introduced intothe device 10 of FIG. 1 The time of contact with the sorbent bed 14 is<0.1 second. The adsorption process is suitably terminated when 0.1% ofthe concentration of components of the contaminated gaseous stream aredetected in the clean air stream (FIG. 2) by sensor 60. At this moment,the contaminated air stream is rerouted via the second identical device58 located in a parallel duct (FIG. 2). This is accomplished by closingthe access valve to the duct containing the device in use (56) andsimultaneously opening the valve in the duct leading to the regenerateddevice (58). In this manner, the flow of contaminated gaseous is notinterrupted during the regeneration process. Depending on the nature andconcentration of the components in the contaminated gaseous stream, thesecond gaseous purification system may not be necessary, and thecontaminated gaseous could be vented to the atmosphere during the 20 to45 minutes required to regenerate the sorbent bed 14. Regenerationfrequency could be as much as once per hour thus allowing the gaseouspurification system to react to varying concentrations of contaminantsin the gaseous stream.

Desorption process

Prior to initiating desorption, the gaseous purification system issealed off with valves which allow the chamber 12 to reach <0.1 mmHgwithout excessive leakage. The chamber 12 is then rapidly evacuateduntil an appropriate vacuum is reached (requires 5 minutes or less). Theelectrodes 16 and 18 are then positioned and current applied to thesorbent bed 14 until it reaches the desired temperature (300 to 500 C;requires 1 minute or less). The wattage required to achieve thistemperature is relatively low(<20 watts). This high temperature/lowpressure treatment is continued for 20 to 45 minutes (depending on thebed depth, contaminant content and temperature). The contaminantsevacuated from the chamber are routed to a recovery container 72. It isnoteworthy that the temperature of the outer wall of the chamber 12never exceeds 40 C and does not require that any special safety measuresbe implemented. Once the heating/vacuum period is completed, the currentto the sorbent bed 14 is turned off and the latter is allowed to cool to<100 C (15 minutes or less) before turning the vacuum pump 74 off andallowing air to gain access to the chamber 12.

With one specific contaminant, toluene, the process ofadsorption/regeneration was repeated some 50 times over a six-monthperiod with no effect on the adsorptive capacity and physical integrityof the sorbet bed 14.

Contaminant recovery

The gaseous contaminants electrothermally desorbed from the sorbent bedduring the regeneration cycle are pumped by a vacuum (50-70Torr)/pressure (20-60 psig) pump directly into a recovery container 72maintained at low pressure (<400 Torr) (FIG. 3). Once the regenerationphase is completed, the access valve 78 to the recovery container 72closes automatically. The piston 80 is repositioned to maintain the lowpressure in the container 72 (<400 Torr). After a number of regenerationcycles (determined by the size of the container 72) and at a point whenthe piston 80 has reached its lowest point, the piston 80 is activatedto compress the volume of the recovery container 72 to an internalpressure required to cause most of the gaseous contaminant to condenseto the liquid phase. The outlet 82 is then opened to release theliquefied contaminant into a waste disposal container. The piston 80 isthen returned to a position that resets the internal pressure ofcontainer 72 to <400 Torr, ready for the next regeneration cycle.Alternatively, the vacuum pump 74 could be eliminated and the process ofcreating an adequate vacuum within the assembly 70 could be achieved bythe motion of the piston 80 within the container 12.

APPLICATIONS OF TILE INVENTION.

While reference has generally been made to gaseous streams bearingcontaminant, in most cases the gaseous will be air, and will be furtherdescribed by reference to contaminated air. Nevertheless, the inventioncan be employed for removal of contaminants from gases other than air,provided that such gases are inert to the processes of adsorption anddesorption of the invention.

The invention described herein can be applied equally well to a systemcharacterized by a continuous air stream flow or one which features aperiodic, continual or interrupted air flow.

1) Continuous Air Flow System

An excellent example of this type of system is the HVAC system found inoffice towers. The air flow is continuous and its contaminant content iscomplex in nature and low in concentration. Odor control is a majorrequirement for such a system. HVAC systems have been identified as amain cause of indoor air quality complaints in office buildings.

The invention described above provides for one of two possibilities.That is, the air purification system (i.e. the adsorption/desorptiondevice) could be integrated as part of the main ventilation system orsmaller units could be easily retrofitted in the main air exhaust ductsconveying return air to the main system from each floor. Both types ofair purification systems allow for a greater amount of airre-circulation since the contaminated return air is effectively cleansedof its gaseous, odoriferous pollutants. It is anticipated that aproperly designed air purification system, based on the inventiondescribed herein, would effectively control odors and gaseouscontaminants during a given workday. The regeneration of the sorbentcould be scheduled after hours. The air purification system would thenbe ready for use for the next workday. The cost of retrofitting an airpurification system and operating it would be recovered throughsignificant reduction in the cost of heating/cooling the air supplied tothe building. Such a system would also effectively address complaints ofpoor indoor air quality by the occupants of the building. It should benoted that such an air purification system would be amenable for use inother enclosed spaces served by a continuous air flow such as aircraftcabins, trains, metro cars, vehicles and private residences.

2) Periodic Air Flow

A good example of this type of air flow is personal respirators worn byworkers using solvents and/or exposed to other gaseous contaminantsduring their workday. One major insurmountable problem with such gaseouscontaminant respirators, to date, is their inherent lack of warning oftheir sudden loss of efficiency leading to the worker wearing them beingplaced at risk of injury and even death. Recent legislation in the USAand Canada requires this problem to be corrected. The inventiondescribed herein would eliminate the problem by incorporating the airpurification system in a removable cartridge of the respirator. Themodifications required to accomplish this are readily achievable andwould allow for the “in-situ” regeneration of the sorbent of thecartridge after each instance of usage. To achieve this, each cartridgeis connected to a source of vacuum and electricity and the temperatureapplied to its sorbent attained by adjusting the electrodes and/orincreasing/decreasing the electric current applied to them in accordancewith the invention. The temperature of the sorbent during theregeneration cycle (typically 30 minutes) is monitored with athermocouple positioned in the sorbent bed. Each respirator cartridge isthen returned to its initial adsorptive capacity after each usageassuring the safety of the workers who wear them. The status of thesorbent bed can be monitored by observing changes in its resistivity.

3) Other Applications

It is one of the great advantages of this invention that it can beintegrated in virtually all types of systems where the removal ofgaseous contaminants from an air stream is required. In this sense, itdoes not suffer from limitations in its applications inherent inprevious art. For instance, the invention described herein could be usedbeneficially to eliminate gaseous contaminant emissions from industrialstacks and for the recovery of valuable solvents released by chemicalprocesses.

NOTABLE FEATURES OF THE INVENTION

One of the outstanding features of the invention described herein is therapid heating of a carbon cloth loaded with adsorbent contaminants toelevated temperatures under high vacuum with the resultant rapid andeffective desorption of the contaminants and regeneration of theactivated carbon cloth “in-situ” in a relatively short period of time(typically <30 minutes).

Another remarkable feature possessed by this invention is the ability itaffords to effectively recover most, if not all, of the contaminantsdesorbed during the regeneration process, an ability which is due, inlarge part, to the relatively small volumes of evacuated air handled(essentially the volume of the chamber).

Furthermore, the sorbent exhibits a long life and conserves essentiallyall of its original adsorptive capacity over a lengthy period of usage,involving repeated adsorption and desorption steps.

The period of time between regeneration of the sorbent bed for this typeof air purification system is dictated by the depth of the sorbent bed,the adsorptive capacity of the carbon cloth of the bed, theconcentration of gaseous contaminants in the air stream, and rate of airflow. Since the sorbent bed depth allowed by the invention is onlylimited by the capacity of the air moving equipment of the host system,the period of time between regeneration can be as lengthy as onedesires. It should be noted that, this is one of the major limitationsof carbon cloth sorbent beds appearing in prior art to date.Essentially, they are limited to a relatively small sorbent bed depthbecause of the configuration adopted.

The power utilized (watts) to heat the carbon cloth to effect itsregeneration is also much lower than that proposed by prior art for tworeasons: (1) the duration of the regeneration cycle (typically 30minutes or less) and (2) the manner in which the electric current isapplied to the carbon cloth sorbent bed, that is, in the preferred “a”direction (from the top to the bottom of the carbon cloth or fromface-to-face through the thickness of the cloth) rather than the moreresistive “b” direction (from one end to the other of the carbon clothi.e. edge-to-edge.).

Another feature of this invention is that it allows for an affordable,cost-effective, rapid retrofitting in a large variety of airpurification systems compared to prior art.

The preferred configuration of the carbon cloth sorbent bed also allowsa designer greater latitude in pre-selecting the pressure drop desiredacross it. This configuration allows for the pressure drop to beminimized by appropriate selection of the weave of the cloth andporosity and the beneficial use of conductive metal spacers within thesorbent bed itself.

The life expectancy of a given carbon cloth sorbent bed described herein(that is, until its replacement is required) is estimated at greaterthan six months based on normal usage.

Since the regeneration process is conducted in a high vacuum (typicallyless than 10 torr and preferably less than 0.51 torr), temperaturesapproaching 1000° C. could be used with attendant shortening of theregeneration period and increased effectiveness of the regenerationprocess.

LIMITATIONS OF THE INVENTION

A possible limitation of the invention described herein is one thatplagues all carbon-based sorbent beds, that is, the reaction between thecarbon and corrosive gaseouses such as HF if present in the air streambeing purified. This leads to the formation of fluorocarbons at hightemperatures. However, since the regeneration process is conducted in ahigh vacuum for a relatively short period of time, it is likely thatthis will not be a problem.

A possible design limitation is the requirement for a sorbent bedchamber having non-conductive walls. This potentially could limit theoverall dimensions of the air purification system that could be designedbased on the invention described. However, preliminary trials based onthe application of a ceramic paste to the inner surface of a stainlesssteel chamber and its subsequent baking at elevated temperaturespromises to remove this limitation.

Laboratory Results

FIG. 4 demonstrates how the power consumption (watts) and the vacuum (mmHg) measured in the carbon sorbent bed chamber decrease concomitantlywith time during the regeneration procedure conducted at 301° C. for 30minutes. Readings were taken at 1 minute intervals and temperature wasmaintained at 301±2° C. The power consumption is reduced by one third ofits initial value. It is matched by an almost ten-fold reduction of theambient pressure in the chamber. Both of these reductions are mostlikely related to the removal of the adsorbate (toluene) from the carboncloth. The attainment of both relatively low power consumption andchamber pressure could be used as indicators of the status of theregeneration process. This result clearly demonstrates the relativelylow power consumption required to achieve complete regeneration of thecarbon cloth sorbent.

FIG. 5 exemplifies the relationship between the equilibrium of thecarbon cloth sorbent bed temperature and the amount of adsorbate(toluene) removed in a 30-minute period. A linear relationship isobtained (r²=0.9985) to 300° C. It demonstrates that the amount ofcontaminant removed is directly dependent upon the temperatureelectrothermally applied to the sorbent bed. In these tests, 100%regeneration was achieved at 300° C. under an average ambient pressure(chamber) of 0.26 mmHg (0.26 torr). This result is obtained at alltemperatures greater than 300° C. in this period of time. The resultsshow the relationship:% toluene desorbed=7.2+0.31×desorption temperatureThese results clearly demonstrate the rapidity of the regenerationprocess conducted as described by this invention.

FIG. 6 shows the relationship between the amount of adsorbate (toluene)removed from the carbon cloth sorbent and the duration of theregeneration process (desorption) at an equilibrium temperature of 300°C. under an average sorbent chamber vacuum of 0.5 mmHg (0.5 torr). It isnotable, that under these conditions, greater than 70% regeneration wasachieved in five minutes, and that greater than 80% of adsorbate isremoved in 20 minutes or less. This result clearly demonstrates that anacceptable amount of regeneration (80% or more) could be achieved usingthe process described herein in relatively short time. Variations in theduration of the regeneration process also would not have a significantimpact on the percent of regeneration achieved.

The relationship may be formulated as:% toluene desorbed=74.9+0.81×desorption time A linear relationship isobtained (r²=0.78).

1. A method for removing a contaminant from a gaseous stream bearing thecontaminant comprising: i) flowing the gaseous stream bearing thecontaminant into a chamber housing a resilient compressible electricallyconductive, activated carbon cloth material, ii) adsorbing saidcontaminant on said cloth material with formation of a carbon clothmaterial loaded with said contaminant, and a gaseous stream liberated ofsaid contaminant, and removing said gaseous stream from said chamber;iii) discontinuing the flow of gaseous stream bearing the contaminantinto the chamber, placing said chamber under vacuum and passing anelectric current through said carbon cloth material loaded with saidcontaminant to generate heat in said carbon cloth material effective todesorb said contaminant from said carbon cloth material, whilemaintaining said vacuum; and wherein said activated carbon clothmaterial in step i) comprises a plurality of layers of electricallyconductive activated carbon cloth, wherein each layer of said pluralityof layers is in electrical contact with at least an adjacent layer ofsaid plurality of layers, and said plurality of layers is disposedbetween, and in electrical contact with. first and second spaced apartelectrodes, and step iii) comprises altering the level of compression ofsaid material to vary the generation of heat to a desired level fordesorption of said contaminant from said carbon cloth material.
 2. Amethod according to claim 1 including: iv) exhausting the contaminantdesorbed from said cloth material in step iii) from said chamber undersaid vacuum.
 3. A method according to claim 1 wherein said vacuum isless than 1.0 torr.
 4. A method according to claim 1 wherein said heatgenerated in step iii) is effective to establish a temperature of 300 to500° C. in said carbon cloth material, and the desorption is completedin 20 to 45 minutes.
 5. A method according to claim 2 including a stepof collecting the desorbed contaminant exhausted from the chamber instep iv).
 6. A method according to claim 1 wherein the desorption of thecontaminant from the carbon cloth material loaded with contaminant instep iii) regenerates the activated carbon cloth material and theregenerated carbon cloth material is employed in steps i) and ii) foradsorption of contaminant from a gaseous stream bearing the contaminant.7. A method according to claim 1 wherein said heat generated in stepiii) is effective to thermally decompose the desorbed contaminant intosimple atmospherically dischargeable gases and including a step ofdischarging said simple gases from said chamber to atmosphere.
 8. Amethod of desorbing a contaminant from a resiliently compressible,electrically conductive, activated carbon cloth material loaded withadsorbed contaminant comprising: a) housing said carbon cloth materialloaded with adsorbed contaminant in a chamber, b) placing said chamberunder vacuum, c) passing an electric current through said carbon clothmaterial and generating heat in said carbon cloth material effective todesorb said contaminant from said carbon cloth material, and whereinsaid activated carbon cloth material comrprises a plurality of layers ofelectrically conductive activated carbon cloth, wherein each layer ofsaid plurality of layers is in electrical contact at least with anadjacent layer of said plurality of layers, and said plurality of layersis disgosed between, and in electrical contact with, first and secondspaced apart electrodes; and step c) comprises altering the level ofcompression of said material to vary the generation of heat to a desiredlevel for desorption of said contaminant from said carbon clothmaterial.
 9. A method according to claim 8 including: d) exhausting thedesorbed contaminant from said chamber under said vacuum.
 10. A methodaccording to claim 8 wherein said vacuum is less than 1.0 torr and saidheat generated in step c) is effective to establish a temperature of 300to 500° C. in said carbon cloth material, and the desorption iscompleted in 20 to 45 minutes.
 11. A method according to claim 8 whereinsaid heat generated in step c) is effective to thermally decompose thedesorbed contaminant into simple atmospherically dischargeable gases andincluding a step of discharging said simple gases from said chamber toatmosphere.
 12. A device for removing a contaminant from a gaseousstream bearing the contaminant comprising: a) a chamber housing aresiliently compressible, electrically conductive, activated carboncloth material; b) said chamber housing a first port for introductioninto said chamber of a gaseous stream bearing contaminant and a secondport for removal from said chamber of the gaseous stream liberated ofthe contaminant; c) a pair of spaced apart electrodes in said housing,said electrodes being electrically in contact with said carbon clothmaterial for flow of electric current between said electrodes andthrough said carbon cloth material, said carbon cloth materialgenerating heat on passage of said electric current therethrough, d)said chamber being gas-tight to support a vacuum therein during thegeneration of heat by the carbon cloth material, and wherein saidmaterial comprises a plurality of layers of electrically conductiveactivated carbon cloth, wherein each layer of said plurality of layersis in electrical contact at least with an adjacent layer of saidplurality of layers, and said plurality of layers is disposed between,and in electrical contact with, first and second spaced apartelectrodes; and further including means to vary the compression of saidmaterial to vary the generation of heat in said material on passage of aconstant electric current therethrough to a desired level for desorptionof contaminant from the carbon cloth material when loaded with adsorbentcontaminant.
 13. A device according to claim 12 further including meansto establish a vacuum in said chamber.
 14. A device for desorbing acontaminant from a resiliently compressible, electrically conductive,activated carbon cloth material loaded with adsorbed contaminantcomprising: i) a chamber housing said carbon cloth material, ii) a pairof spaced apart electrodes in said housing, said electrodes beingelectrically in contact with said carbon cloth material for flow ofelectric current between said electrodes and through said carbon clothmaterial, said carbon cloth material generating heat on passage of saidelectric current therethrough, iii) said chamber being gas-tight tosupport a vacuum therein during the generation of heat by the carboncloth material, iv) an exhaust port for exhausting under vacuum acontaminant desorbed from the carbon cloth material by the heatgenerated by the carbon cloth material, and wherein said materialcomprises a plurality of layers of electrically conductive activatedcarbon cloth, wherein each layer of said plurality of layers is inelectrical contact at least with an adjacent layer of said plurality oflayers, and said plurality of layers is disposed between, and inelectrical contact with, first and second spaced apart electrodes; andfurther including means to vary the compression of said material to varythe generation of heat in said material on passage of a constantelectric current therethrough to a desired level for desorption ofcontaminant from the carbon cloth material when loaded with adsorbentcontaminant.
 15. A device according to claim 14 further including meansto establish a vacuum in said chamber.
 16. A device according to claim14 further including container means communicating with said exhaustport for collection of desorbed contaminant.